Superconducting cable

ABSTRACT

A superconducting cable particularly suitable for use as an alternating-current cable having at least one coaxial pair of conductors which are arranged in a coolant pipe, with the respective tubular inner and outer conductors being separated from each other by a plastic material insulation. The plastic insulation is designed as a tubular carrier that is flexible at room temperature, and the inner and outer conductors each consist of a ribbon made up of both a superconducting metal and an electrically normal-conducting metal which is wound in a singlelayer helix form. The helix forming the inner conductor is in contact with the inside of the plastic tubular carrier while the helix forming the outer conductor is wound on the outside of such plastic tubular carrier. The cable can be assembled from prefabricated sections.

United States Patent Bogner et al.

SUPERCONDUCTING CABLE Inventors: Gunther Bogner, Tennenlohe; Fritz Schmidt, Erlangen, both of Germany Siemens Aktiengesellschatt, Munich, Germany Filed: Mar. 3, 1972 Appl. No.: 231,669

Assignee:

Foreign Application Priority Data Mar. 10,1971 Germany P 21 11515.1

US. Cl 174/15 C, 174/126 CP, 174/27, l74/D1G. 6

Int. Cl H0lv 11/00 Field of Search 174/15 R, 15 C, D16. 6, 174/28, 27, 29, 126 R, 126 CP, 128; 335/216 References Cited UNITED STATES PATENTS [451 July 31, 1973 3,529,071 9/1970 Kafka 174/15 C Primary Examinerl-larold Broome Assistant Examiner-A. T. Grimley Attorney-Hugh A. Chapin [57] ABSTRACT A superconducting cable particularly suitable for use as analternating-current cable having at leastone coaxial pair of conductors which are arranged in a coolant pipe, with the respective tubular inner and outer c0nductors being separated from each other by a plastic material insulation. The plastic insulation is designed as a tubular carrier that is flexible at room temperature, and the inner and outer conductors each consist of a ribbon made up of both a superconducting metal and an electrically normal-conducting metal which is wound in a single-layer helix form. The helix forming the inner conductor is in contact with the inside of the plastic tubular carrier while the helix forming the outer conductor is wound on the outside of such plastic tubular carrier. The cable can be assembled from prefabricated sections.

11 Claims, 8 Drawing Figures SUPERCONDUCTING CABLE BACKGROUND OF THE INVENTION layer, are commonly arranged with the evacuated space to provide further thermal insulation.

In order to obtain the best possible thermal insulation I of the coolant pipe against the environment of the cacoolant pipe and may be assembled from prefabricated sections.

2. Description of the Prior Art It is known to arrange in superconducting cables one or several tubular, coaxial pairs of conductors in a coolant pipe. In three-phase cables, for instance, three such coaxial pairs of conductors can be provided. Here,-the inner conductor of each conductor pair serves as the outgoing conductor and the outer conductor as the return conductor, which generally is at ground potential. The phases are linked outside of the cable proper. The electromagnetic field is formed in such coaxial conductor pairs only in the space between the inner conductor and the outer conductor, while the other areas remain field-free. For insulating the inner conductor from the outer conductor, it is known to evacuate the space between the two conductors, or to fill it with liquid helium, which then serves as the insulating means. Furthermore, a proposal for insulating the space between the two conductors with plastic material is also already known, which consists of thin films of stabilized polyethylene, with which the inner conductor is covered, as disclosed in the publication Elektrotechnische Zeitschrift, Series B. vol. (1968), pp. 273 to 277, particularly at p. 275.

The inner conductor and the outer conductor of a coaxial conductor pair consist in these known cables of rigid tubes, preferably of an electrically normalconducting metal of high electrical conductivity, such as very pure copper or aluminum, onto which the superconducting material is applied in the form of a layer. Pure niobium and lead have been found to be particularly suitable superconducting materials for 50 Hz alternating current because these superconducting materials have very low hysteresis losses, provided that in the operation of the cable a lower critical magnetic field limit H is not exceeded. In order to avoid eddy current losses arising in the normal-conducting tubes which support the superconducting layer, the latter is arranged on the outside of the inner tube and on the inside of the outer tube of the coaxial conductor pairs. During the operation of the superconducting cable, especially if the conductor pairs are insulated by vacuum or plastic material, a coolant, such as liquid helium, flows in the coolant pipe within'the tubular inner conductor and on the outside of the tubular outer conductor. The inner and outer conductors are in direct contact with the coolant and are cooled substantially thereby. As a general rule, the coolant pipe itself is surrounded by a tubular radiation shield, which is cooled, for instance, by nitrogen, and this shield is in turn surrounded by an outer protective tube. The space between the coolant pipe and the outer protective tube is evacuated during the operation of the cable for the purpose of thermal insulation. Thin plastic films, which may, for instance, be coated with a reflecting metallic ble, the coolant pipe, the radiation shield and the outer protective tube are made of rigid tubes, as is well known. Such rigid tubes can be supported against each other by means of relatively few spacers of material with low heat conductivity. In the installation of the superconducting cable, these rigid tubes are assembled at the point of installation from individual sections. Here, expansion-equalizing sections, and in particular, corrugatedsections, can be provided for equalizing thedif. ferent material shrinkages when cooling down, particularly in the coolant pipe and in the radiation shield.

The inner and outer conductors of the individual coaxial conductor pairs, which consist of rigid tubes, must also be assembled from sections at the point of installation. For shipping reasons, the length of these sections should not exceed 20 to 50 meters. This leads to a mu]- tiplicity of joints, at which the superconducting layers situated on the sections must be connected with each other, for instance, by welding. Besides the large technical effort required therefor, the probability of defective welds increases, of course, with an increasing number of welds, with the consequent probability of an impairment of the current-carrying capacity of the superconductors. Furthermore, the expansion compensation presents considerable difficulties with such rigid tube coated with superconducting material.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a superconducting cable including'at least one coaxial pair of conductors which are arranged in a coolant pipe, may be assembled from prefabricated sections, and with the respective tubular inner and outer conductors being separated from each other by insulation of plastic material, in such a manner that the conductor pairs can be wound on drums and be brought to the point of installation in lengths as large as possible.

It is another objectto .provide a superconducting cable wherein the design of the conductor pairs is simple, and the advantages of direct cooling, which exist with rigid, tubular conductors, are retained.

These and other objects are-achieved by the present invention which provides a superconducting cable having at least one coaxial pair of conductors separated from each other by insulation of plastic materials, wherein such plastic material insulation is designed as a tubular carrier which is flexible at room temperature, and the inner and outer conductors each consist of ribbon which is made up of a superconducting metal and an electrically normal-conducting metal which is wound into a single-layer helix. The helix forming the inner conductor makes contact with the inside of the tubular carrier, and the helix forming the outer conductor is wound on the outside of the tubular carrier.

A conductor pair rnadeaccording to the present in.- vention can be manufactured in long sections of, for instance, several hundred meters, and can be transported to the place of installation where they are inserted into the coolant pipe of the cable. If the cable is so long that the conductor pairs must be assembled from prefabricated sections, which will generally be the case, the number of joints along the cable at which the superconductors must be connected in the installation, is considerably reduced due to the great length of the sections. Since the plastic insulation is designed as a carrier tube, a simple mechanical construction is obtained, and sufficient mechanical stability of the conductor pair is assured at the same time. The inner and outer conductors, consisting respectively of a single-layer helix, are sufficiently flexible and are held mechanically by the plastic carrier tube. The helixes are directly wetted by the coolant, which during the operation of the cable flows through the interior of the tubular carrier and along its outside, and are therefore cooled exceedingly well.

Niobium, and also lead, are suitable for use as the superconductor material for the a-c cable. Copper and aluminum are suitable as the electrically normalconducting metals which provide for the electrical stabilization of the superconductors. Electrically normalconducting metals are understood in this context to be metals which have normal electric conductivity at the operating temperature of, for instance, about 4 to 5 K, which is necessary to bring about the superconducting state.

The ribbons forming the inner and outer conductors consist of a superconducting and an electrically normal-conducting layer, the superconducting layer always facing the tubular carrier. This arrangement is advantageous in that the normal-conducting layers are outside the space occupied by the field between the superconductors, thereby avoiding a-c losses in the normal-conducting metal. In order to avoid losses due to irregularities of the magnetic field at the edges of the ribbon, the helixes are, in addition, wound in such a manner that the ribbon edges of the adjacent turns of each helix lie closely together or overlap slightly with their superconducting layers, even after being cooled down to the low operating temperature of the cable which is necessary to bring about superconductivity. To achieve such overlap, the superconducting layer extend at one ribbon edge over the normal-conducting layer and be set back at the other ribber edge with respect to the normal-conducting layer. As the helixes can spread in the lengthwise direction upon cooling down, expansion compensation is readily possible.

In selecting the plastic material for the insulating carrier tube, a number of considerations must be taken into account. The plastic materials should, first of all, have a dielectric strength as high as possible so that the wall thickness of the carrier tube and thereby the diameter of the outer conductor can be kept as small as possible. While the diameter of the inner conductor must be so large that the maximum permissible magnetic field at the surface of the inner conductor is not exceeded, the diameter of the outer conductor depends essentially on the dielectric strength of the insulating carrier tube. A small outer-conductor diameter facilitates winding the conductor pairs on drums and further permits the diameter, and therefore the surface, of the coolant pipe and the other tubes of the cable to be maintained small, which in turn leads to a reduction of the thermal losses which are proportional to the size of the cable surface,

While the dielectric losses generated in the insulating carrier tube must be removed through the liquid coolant, these losses should also be maintained as small as possible. The tangent of the loss angle 5 of the plastic materials used for the carrier tube should therefore be in the order of about at a temperature of about 4 to 5 K, or even smaller. The plastic should furthermore have a low dielectric coefficient. It is also advantageous if the elastic tensile stress limit of the insulating carrier tube is higher than the tensile stress that occurs if the carrier tube is cooled down to the operating temperature, at which pointthe tensile stress is equal to the product of Young's modulus and the contraction. Since the carrier tube is deformed only elastically upon cooling, if this requirement is met, it becomes possible to clamp the conductor pairs firmly at the cable ends or at the ends of the conductor pair sections, so that no further provisions-for expansion compensation are necessary. The tubular plastic carrier of the conductor pair section can advantageously consist of a single continuous piece. Suitable plastic materials are, for example, polyethylene, polytetrafluorethylene and polyamides, such as nylon.

After cooling down to the low operating temperature of the cable, no cracks should further occur in the insulating carrier tube, which would otherwise lead to a reduction of the dielectric strength. Although the already-mentioned plastic materials have no tendency to form cracks under normal conditions, it is nevertheless advantageous, for safety reasons, to build up the tubularcarrier from two coaxial plastic tubes which are flexible at room temperature, and a multi-layer winding of plastic film arranged between these tubes. The two coaxial plastic tubes, which can be kept relatively thin, also provide the mechanical stability, while high dielectric strength is obtained by the multi-layer winding of plastic film. It is particularly advantageous if the two plastic tubes are imperious to the coolant, so that no coolant can penetrate into the insulation and the formation of coolant vapor bubbles within the insulation is prevented. For this purpose, it is preferable to use plastic tubes of nylon braid which have been densified by a heat treatment, while the plastic film wound in between the plastic tubes consists of polyethylene fiber or polytetrafluorethylene fiber paper.

In instances where it is not important that the tubular carrier is impervious to the coolant, the'tubular carrier of a conductor pair section can consist of several joined-together, mechanically engaging tube sections. Through such a design, relief of the tubular carrier of tensile stresses occuring upon cooling down can be obtained.

Joining of the individual conductor pair sections is accomplished in a simple manner whereby the outside diameter of the tubular carriers is reduced at the junctions between two conductor pair sections at their ends and a section of tubing is inserted into the latter for the purpose of joining the tubular carriers. The inner conductors of the conductor pair sections are then connected with each other at the outside of this section of tubing. Several insulating sleeves of half-shells are placed over the joint and the tubular carrier ends with their reduced outside diameters, these sleeves being placed on top of and displaced against each other, for the purpose of insulation. The outer conductors of the conductor pair sections are placed around these sleeves from the outside and are joined with each other. The

tube section inserted into the tubular carriers may, for

of the two conductor pair sections can then be connected with the superconductor material on the outside of the section of tubing, by a method such as welding. With this type of construction, electrical stabilization as well as direct cooling of the joint is assured.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows schematically in cross section a superconducting three-phase cable with three coaxial conductor pairs;

FIG. 2 shows schematically a section of a coaxial conductor pair for a cable according to FIG. 1, illustrative of onev embodiment of the invention; I k

FIG. 3 shows a longitudinal cross section view of a coaxial conductor pair, illustrative of another embodiment of the invention;

FIG. 4 shows a longitudinal cross section view of a further embodiment of a coaxial conductor pair;

FIG. 5 shows a longitudinal cross section view of a joint between two conductor pair sections;

FIG. 6 shows a transverse cross section view along the line AB indicated in the cable shown in FIG. 5; b

FIG. 7 shows one manner of joining the ends of the helical conductors together; and p FIG. 8 shows schematically a section of a tubular carrier with an inner conductor having overlapping ribbon edges.

DESCRIPTION OF THE PREFERRED EMBODIMENTS from the contraction of the conductor material. For example, a ribbon-shaped conductor which is comprised of a copper layer and asuperconducting niobium layer,

7 contracts by about 0.32 per'centwhen cooled from FIG. 1 shows the basic construction of a superconducting cable with several coaxial conductor pairs constituting a three-phase cable. In a coolant pipe 1, which may consist, for instance, of high-grade alloy steel, are located three coaxial conductor pairs 2, 3 and 4. Each coaxial pair consists of a tubular plastic carrier 5, the inside of which is in contact with the inner conductor 6 wound in the form of a helix and on the outside of which is helically wound an outer conductor 7. In the operation of the cable, a coolant 8, in particular, liquid helium, flows through the free interior'of each coaxial conductor pair and along the outside of the coaxial conductor pairs. The coolant pipe 1 is surrounded on the outside by a radiation shield 9, made of a material such as copper, which is cooled by liquid nitrogen which flows through the pipe 10. The radiation shield 9 is in turn enclosed by a protective tube 11, madeof a material such as high-grade alloy steel. The space between the coolant pipe 1 and the protective tube 11 is evacuated in the operation of the cable.

A section of a coaxial conductor pair for a cable according to FIG. 1 is shown schematically in FIG. 2. It can clearly be seen from FIG. 2 that the inner conductor 6 and the outer conductor 7 each consist of a ribbon which is wound to form a single-layer helix The inner conductor 6 is in contact with the inside of the tubular carrier 5, while the outer conductor 7 is wound on the outside of the tubular carrier 5. In the manufacture of the coaxial conductor pair, one can, for example, proceed in such a manner that the tubular plastic carrier 5 is produced first, for example, of polyethylene, the inner conductor 6, wound to form a helix, is inserted into the tubular carrier 5 and then the outer conductor 7 is wound around such tubular carrier 5. However, it is noted that the inner conductor 6 can be first wound into a helix, after which this helix is then surrounded by the tubular carrier 5 by, for example, an extrusion proroom temperature to the temperature of liquid helium.

A particularly advantageous design of a coaxial conductor pair is shown in longitudinal cross section in FIG. 3. In this embodiment, the tubular plastic carrier consists of two coaxial plastic tubes 31 and 32, which are flexible at room temperature. Between the tubes 31 and 32, is wound a multi-layer wrapping 33 of plastic film. The plastic tubes 31 and 32 are made of nylon, which shrinks byonly about I percent when cooled from room temperature to about 4.2 K. The tube31, situated inside, can be manufactured by covering the inner conductor 34, already wound into a helix, with a multi-layer braid of nylon threads. After the braiding, the nylon braid can be made impervious to the coolant by heating it at its surface for a short period to a'temperature of about 150 C. For this purpose the inner conductor 34, covered with the nylon braid, can be pulled through a tubular furnace. Subsequently, a multiplicity of layers of plastic film, made of polyethylene fiber paper or polytetrafluorethylene fiber paper, is wrapped around the tube 31. In this connection, care should be taken, as indicated in FIG. 3, that the joints between the windings of one layer are covered by a turn of the next layer, so that no short paths void of plastic film occur between the tubes 31 and 32. To make the plastic tube 32, the wrapping '33 is subsequently covered with a nylon braid of several layers, which is likewise densitied at its surface by a heat treatment. The helical outer conductor 35 is then wound around the tube 32. The inner conductor 34 and the outer conductor 35 each consist of a copper ribbon 36 and 37, respectively, each of which is coated on one side with a niobium layer 38 and 39, respectively. The superconducting layers 38 and 39 respectively face the tubular carrier. When cooled from room temperature to the temperature of liquid helium, the polyethylene filmshrinks by by about 2.6 percent and the polytetrafluorethylene film by about 2.2 percent, i.e., somewhat more than the nylon tubes. However, if the tubes 31 and 32 and the films 33 are bonded together at the ends of the tubular carrier, by cementing or a suitable heat treatment, a simple compensation for the expansion is obtained due to the'fact'that the individual-turns of the I film wrapping 33 spread somewhat when cooled down;

The tubes 31 and 32 can, for example, be about 1 mm thick, while the film wrapping 33 has a total thickness of l to 2 cm and may consist of a multiplicity of, for instance, (micron) thick plastic films. The outside diameter of the inner-conductor helix can here be about 6 cm, while the copper ribbon36 and 37, respec-' tively, of the inner and outer conductor is about 2 mm thick and the niobium layers 38 and 39 are about 0.5 mm thick. In addition to being impervious to the coolant, the tubes 31 and 32 protect the plastic winding 33 located between them against moisture and other atmospheric influences in case of extended storage, and also during the installation of the cable outdoors.

FIG. 4 shows in longitudinal cross section another design of a coaxial conductor pair, wherein the tubular carrier of a conductor pair section consists of several joined-together, mechanically engaged plastic tubing sections 41, 42 and 43. Through such a design the tubular plastic carrier is relieved of tensile stresses when cooled down. At one end of each section is provided an annular extension 44, which engages with an annular groove 45 found at the other end of each section. The extension 44 should be as long as possible and the groove 45 as deep as possible so that the path 46 that is free of insulating material between the inner conductor helix 47 and the outer conductor helix 48 is made as long as possible. By slightly bevelling the walls of the groove 45, flexibility of the tubular carrier can be increased in the overlap region of the tube sections.

As already mentioned, the ends of the individual conductor pair sections must be connected together at the joints if the cable length requires it. The preferred manner of making these joints is shown in FIGS. to 7. More particularly, FIG. 7 shows a joint between two conductor pairs in longitudinal cross section, and FIG. 6 shows a cross section through FIG. 5 along the line AB. The outside diameters of the tubular carriers 51 and 52 of the two conductor pair sections are reduced at both ends 53 and 54. In order to connect the two ends, a section of tube 55 is first inserted into them in such a manner that it comes to lie with its ends within the inner-conductor helixes 56 and 57. The tube section 55 is composed of two concentric layers, the outer layer 58 consisting of niobium and the inner layer 59 consisting of copper which serves for the electrical stabilization of the niobium. To join the inner conductors 56 and 57 together electrically, the stabilizing copper 62 is first removed at the ends 60 and 61 of the inner conductors, so that the conductor ends consist only of niobium 63. Subsequently the conductor ends 60 and 61 are placed on the outside around the tube section 55 and are connected to the latter's outside layer 58, which also consists of niobium, by a method such as welding. Around the joint is then placed a plastic sleeve which consists of two half-shells 64 and 65. Around this sleeve and the reduced diameter ends 53 and 54 of the tubular carriers 51 and 52 is then placed a second plastic sleeve consisting of two half-shells 66 and 67. Referring to FIG. 6, the gaps between the two half-shells of each sleeve are advantageously displaced against each other, so that the free gaps between the inner conductor and the outer conductor of the cable become as long as possible. The ends 68 and 69 of the helical outer conductors of the two conductor pair sections are then placed around the sleeve consisting of the halfshells 66 and 67.

One manner of joining the ends 68 and 69 together is shown in FIG. 7. Stabilizing copper 70 is first removed at the area shown so that the superconducting niobium ends 71 are exposed. The niobium ends 71 of the two conductors are then welded together at 72. In case the plastic material of the sleeve, which consists of the half-shells 66 and 67 around which the conductor ends 68 and 69 are placed for welding, cannot withstand the welding temperature, inserts 73 and 74 in the form of half-shells of a heat-resistant material such as ceramics or asbestos, can be provided in this sleeve. After welding the niobium ends 71, a copper ribbon 75 is finally welded to he stabilizing copper at 76 and 77, thereby connecting the stabilizing copper 70. Another method of connecting the ends 68 and 69 of the outer conductors consists of providing, in place of the above-described inserts 73 and 74 in the shape of halfshells of heat-resistant material, a sleeve of superconducting material consisting of two half-shells, made of niobium, and connecting the niobium ends 71 ofthe two outer conductors with the outside of this sleeve, by welding. Subsequently, the stabilizing copper 70 of the two conductor ends is again connected together. The 1 joining technique explained abovehas the-particular advantage that it can be executedsimply and that, even at the particularly critical junction points, the direct cooling of the inner conductor and the outer conductor by the coolant remains intact.v

Some important electrical and mechanical data of the plastic materials mentioned above are given in the following Table.

It should be noted regarding the values in the Table that the tensile strength at low temperatures is considerably higher than at the temperature of 293 K, for which the values in the Table are given.

Referring to FIG. 8 there is shown a section of a tubular plastic carrier 81 with a ribbon-shaped innerconductor helix in contact with the inside of such tubular plastic carrier 81, and with the adjacent innerconductor turns overlapping with their superconducting layers. The superconducting layer 82 is displaced relative to the normal-conducting layer 83 of the ribbon in such manner that at one edge of the ribbon it extends beyond the latter and at the other edge is set back with respect to it. Such a ribbon can be manufactured, for instance, by rolling a niobium ribbon 82 on a copper ribbon 83. This kind of overlap provides the advantage that no additional losses of any kind occur in the copper at the edges of the ribbon.

According to the invention, the flexible, coaxial conductor pairs can, of course, also be used in cables in which the coolant pipe, the radiation shield and the outer protective tube are made flexible, such as by the use of corrugated tubing. Also, in such cables, use can be made of the advantageous properties of the conductor pairs, particularly of their great'transportability. In order to obtain sufficient mechanical stability in such cables, however, numerous supports would be required between the flexible. outerjtubes which degrade the thermal insulation. Cables with a rigid coolant pipe, radiation shield and protective tube are, therefore, preferred. i

Although the above description is directed to the preferred embodiments of he invention, it is noted that other variations and modifications will be apparent to those skilled in the art and, therefore, may be made without departing from the spirit and scope of the present disclosure.

What is claimed is:

l. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabricated sections, comprising:

at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubularouter conductor; an insulation of plastic material located between said inner conductorand said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature; and

each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a layer of a superconducting material and a layer of an electrically normalconducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier, and the helix forming said outer conductor being wound on the outside of said tubular carrier, 2. A superconducting cable as recited in claim 1, wherein the ribbons forming said inner conductor and said outer conductor each consist of a superconducting and an electrically normal-conducting layer, the superconducting layer of each conductor is arranged to facethe tubular carrier, and the ribbon edges of the adjacent turns of each helix lie closely together even after cooling down to the low operating temperature of the cable that is required to bring about the superconductivity, or overlap slightly with their superconducting layers.

3. A superconducting cable as recited in claim 1, wherein said said tubular carrier of a conductor pair section comprises a single, continuous piece.

4. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabricated sections, comprising:

at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubular outer conductor;

an insulation of plastic material located between said inner conductor and said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature; and

set back at the other edge of said ribbon with reternating current cable and for assembly from prefabri- 5 cated sections, comprising:

at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubular outer conductor;

an insulation of plastic material located between said inner conductor and said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature, said tubular carrier including two coaxial plastic tubes which are flexi-v ble at room temperature, and a multi-layer wrapping of plastic film which is arranged between said plastic tubes; and

each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a superconducting material and an electrically normal-conducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier, and the helix forming said outer conductor being wound on the outside of said tubular carrier.

6. A superconducting cable as recited in claim 5, wherein said two plastic tubes are impervious to the coolant.

7. A superconducting cable as recited in claim 6, wherein said plastic tubes comprise a nylon braid densified by a heat treatment, and said plastic film comprises a polyethylene fiber or polytetrafluorethylene fiber paper.

8. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabrieach of said inner and outer conductors consisting of a ribbon which is wound to form a single'layer helix and is constituted by a larger of a superconducting material and a layer of an electrically normalconducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier and the helix forming said outer conductor being wound on the outside of said tubular'carrier, the superconducting layer of each conductor is arranged to face the tubular carrier, the ribbon edges of the adjacent turns of each helix lie closely together even after cooling down to the low operat ing temperature of the cable that is required to bring about the superconductivity, or overlap slightly with their superconducting layers, and the superconducting layer extends at one edge of said ribbon beyond the normal-conducting layer and is cated sections, comprising:

at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubular outer conductor;

an insulation of plastic material located between said inner conductor and said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature, the tubular carrier of a conductor pair section consisting of several, joined-together, mechanically engaged tube sections; and

each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a superconducting material and an electrically normal-conducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier, and the helix forming said outer conductor being wound on the outside of said tubular carrier. 9. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabricated sections, comprising:

at least one coaxial pair of conductors arranged in a coolant pipe, said coaxialpair including a tubular inner conductor and a tubular outer conductor;

an insulation of plastic material located between said inner conductor and said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature;

each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a superconducting material and an electrically normal-conducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier, and the helix forming said outer conductor being wound on the outside of said tubular carrier; and

means for joining said sections of cable, including a reduced outside diameter portion at the ends of said tubular carriers where a joint is formed between two adjacent conductor pair sections a tube section inserted into the inside of said tubular carrier between the adjacent ends for the purpose of joining said tubular carriers, said inner conductors of the conductor pair sections being connected with each other at the outside of said tube section, and a plurality of insulating sleeves of half-shells placed over said joint and said carrier ends having reduced outside diameters, said insulating sleeves being arranged on top of each other and displaced with respect to each other, and said outer conductors of the conductor pair sections being placed around the outside of said insulating sleeves and connected with each other.

10. A superconducting cable as recited in claim 9, wherein said tube section inserted into the tubular carrier includes a superconductor material layer on the outside and an electrically normal-conducting metal on the inside, with said superconductor material of the inner conductors of both conductor pair sections being connected with the outside of the tube section.

11. A'super'conducting cable as recited in claim 9, wherein a sleeve of superconducting material, formed of half-shells, is arranged over the outside of said insulating sleeves at said joint; and the superconductor material of the outer conductors of both conductor pair sections is connected with the outside of said sleeve.

* III III 

1. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabricated sections, comprising: at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubular outer conductor; an insulation of plastic material located between said inner conductor and said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature; and each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a layer of a superconducting material and a layer of an electrically normal-conducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier, and the helix forming said outer conductor being wound on the outside of said tubular carrier.
 2. A superconducting cable as recited in claim 1, wherein the ribbons forming said inner conductor and said outer conductor each consist of a superconducting and an electrically normal-conducting layer, the superconducting layer of each conductor is arranged to face the tubular carrier, and the ribbon edges of the adjacent turns of each helix lie closely together even after cooling down to the low operating temperature of the cable that is required to bring about the superconductivity, or overlap slightly with their superconducting layers.
 3. A superconducting cable as recited in claim 1, wherein said said tubular carrier of a conductor pair section comprises a single, continuous piece.
 4. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabricated sections, comprising: at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubular outer conductor; an insulation of plastic material located between said inner conductor and said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature; and each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a larger of a superconducting material and a layer of an electrically normal-conducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier and the helix forming said outer conductor being wound on the outside of said tubular carrier, the superconducting layer of each conductor is arranged to face the tubular carrier, the ribbon edges of the adjacent turns of each helix lie closely together even after cooling down to the low operating temperature of the cable that is required to bring about the superconductivity, or overlap slightly with their superconducting layers, and the superconducting layer extends at one edge of said ribbon beyond the normal-conducting layer and is set back at the other edge of said ribbon with respect to the normal-conducting layer.
 5. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabricated sections, comprising: at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubular outer conductor; an insulation of plastic material located between said inner conductor and said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature, said tubular carrier including two coaxial plastic tubes which are flexible at room temperature, and a multi-layer wrapping of plastic film which is arranged between said plastic tubes; and each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a superconducting material and an electrically normal-conducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier, and the helix forming said outer conductor being wound on the outside of said tubular carrier.
 6. A superconducting cable as recited in claim 5, wherein said two plastic tubes are impervious to the coolant.
 7. A superconducting cable as recited in claim 6, wherein said plastic tubes comprise a nylon braid densified by a heat treatment, and said plastic film comprises a polyethylene fiber or polytetrafluorethylene fiber paper.
 8. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabricated sections, comprising: at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubular outer conductor; an insulation of plastic material located between said inner conductor and said outer conductor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature, the tubular carrier of a conductor pair section consisting of several, joined-together, mechanically engaged tube sections; and each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a superconducting material and an electrically normal-conducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier, and the helix forming said outer conductor being wound on the outside of said tubular carrier.
 9. A superconducting cable suitable for use as an alternating current cable and for assembly from prefabricated sections, comprising: at least one coaxial pair of conductors arranged in a coolant pipe, said coaxial pair including a tubular inner conductor and a tubular outer conductor; an insulation of plastic material located between said inner conductor and said outer conduCtor and separating said conductors from each other, said plastic insulation providing a tubular carrier that is flexible at room temperature; each of said inner and outer conductors consisting of a ribbon which is wound to form a single layer helix and is constituted by a superconducting material and an electrically normal-conducting metal, the helix forming said inner conductor being in contact with the inside of said tubular carrier, and the helix forming said outer conductor being wound on the outside of said tubular carrier; and means for joining said sections of cable, including a reduced outside diameter portion at the ends of said tubular carriers where a joint is formed between two adjacent conductor pair sections a tube section inserted into the inside of said tubular carrier between the adjacent ends for the purpose of joining said tubular carriers, said inner conductors of the conductor pair sections being connected with each other at the outside of said tube section, and a plurality of insulating sleeves of half-shells placed over said joint and said carrier ends having reduced outside diameters, said insulating sleeves being arranged on top of each other and displaced with respect to each other, and said outer conductors of the conductor pair sections being placed around the outside of said insulating sleeves and connected with each other.
 10. A superconducting cable as recited in claim 9, wherein said tube section inserted into the tubular carrier includes a superconductor material layer on the outside and an electrically normal-conducting metal on the inside, with said superconductor material of the inner conductors of both conductor pair sections being connected with the outside of the tube section.
 11. A superconducting cable as recited in claim 9, wherein a sleeve of superconducting material, formed of half-shells, is arranged over the outside of said insulating sleeves at said joint; and the superconductor material of the outer conductors of both conductor pair sections is connected with the outside of said sleeve. 